alpha channel

Ordinary video clips and image files have three channels of color information: red, green, and blue. Many video and image file formats also support an additional alpha channel, which contains information defining areas of transparency. An alpha channel can be represented as a grayscale image, with solid areas (100 percent opacity) shown in white, translucent areas shown in gray, and transparent areas (0 percent opacity) shown in black.

When you import a QuickTime movie or an image file into a project, its alpha channel is recognized by Final Cut Pro. The alpha channel is then used to composite that image against other images in the viewer.